There are two types of methods of producing toner for development used by an electrophotographic image forming apparatus. Specifically, used as such two types of methods are (i) a dry method in which no aqueous medium is used, and (ii) a wet method in which an aqueous medium is used. In general, the dry method indicates a pulverization method, including the steps of (i) mixing a powdery binder resin, a coloring agent, a charge control agent, and a wax, by using a wind-power mixer such as a Henschel mixer, (ii) kneading thus obtained powdery mixture, by using a device such as a twin-screw kneader, (iii) cooling the mixture, and (iv) pulverizing thus obtained solid material of resin by using a mill such as a jet mill, until the solid material of resin is pulverized into several microns.
Exemplary wet methods include suspension polymerization, and emulsion polymerization in combination with aggregation. An exemplary known process of the suspension polymerization is that including the steps of (i) dispersing, in water, a vinyl monomer, which is a raw material of a binder resin, a polymerization initiator, a coloring agent, a charge control agent, a release agent and the like, together with a dispersing agent by using a stirrer such as a homogenizer, and then (ii) performing suspension polymerization, thereby forming toner particles. On the other hand, an exemplary known process of the emulsion polymerization in combination with aggregation is that including the steps of (i) performing emulsion polymerization by using a vinyl monomer, which is a raw material of a binder resin, and (ii) aggregating thus obtained resin dispersion with an aqueous dispersion of coloring agent, an aqueous dispersion of charge control agent, and an aqueous dispersion of wax, thereby forming toner particles.
Meanwhile, in recent years, there has been an increasing demand for an electrophotographic apparatus to be able to produce a color image which is equivalent in quality to a gravure or a photographic picture, as color printing with the use of an electrophotographic image forming apparatus has become developed. Achieving high image quality requires a development technique that realizes high dot-reproducibility without causing unevenness of colors or graininess, as well as a high-resolution scanner and a high image quality image-processing technique. To fulfill the requirement, downsizing of toner particle becomes an important problem to be solved.
In view of downsizing of toner particle, the pulverization requires a longer time and greater energy for pulverizing to produce toner with a small particle size, and therefore has problems that (i) an amount of produced toner decreases and (ii) cost of producing toner increases. Furthermore, a number of contaminating free waxes and contaminating free charge control agents increases, which free waxes and free charge control agents are generated in the step of pulverizing. This has a tendency to often cause a formation of a film on a carrier or a photoreceptor. This makes it difficult to produce toner with a volume average particle diameter of 6 μm or smaller.
On the other hand, the polymerization allows microscopic toner particles to be directly produced in an aqueous medium, and is thus considered as a more suitable technique for producing toner with a smaller particle size than the pulverization. However, suspension polymerization and emulsion polymerization require to use a dispersing agent (surfactant) to disperse a monomer or a particulate resin in an aqueous medium. This causes a highly hydrophilic dispersing agent (surfactant) to remain on a surface of toner. As a result, a property of toner such as an amount of charges or an electric resistance changes in accordance with a change in humidity. Therefore, the polymerization has a problem that stability (humidity-resistance) to condition of humidity decreases.
The highly hydrophilic dispersing agent can be removed to some extent by washing the surface of toner with the use of an enormous amount of pure water. This, however, gives rise to a problem of increasing costs of production and processing waste water. Furthermore, it is impossible to remove a dispersing agent (surfactant) remained in the toner particles. This results in that the amount of absorbed water caused by the residual dispersing agent (surfactant) in the toner (circumference) changes depending upon the degree of dryness of the toner, thereby causing an amount of charges and an electric resistance of the toner to be unstable.
In view of the problems, there is suggested a method of producing toner without using a highly-hydrophilic dispersing agent (surfactant) in Patent Document 1 (Japanese Unexamined Patent Publication No. 2001-356528 (published on Dec. 26, 2001)).
According to the method of Patent Document 1, toner is produced by (i) applying shear stress, in a hydrophobic organic medium to which the resin component is insoluble, to (a) a pigmented resin composition including a resin, a coloring agent, and a charge control agent, and (b) a copolymer of polyvinylpyrrolidone and eicosyne (polymer dispersing agent) so that the composition becomes particles to form toner particles, and then (ii) separating the toner particle from the organic medium.
The method of Patent Document 1 does not require to use a highly-hydrophilic dispersing agent, and therefore can efficiently produce toner that is small in particle size and excellent in humidity-resistance.
However, the resin needs to be heated to a temperature equal to or above a softening point of the resin, in order for the resin composition to become particles. Therefore, there is a problem that, if a resin with a greater molecular weight and a higher softening point is used in order to improve offset resistance, the resin, the additive, and/or other components may be thermally decomposed while they are heated. Furthermore, the method disclosed in Patent Document 1 has at least the following difficulty in handling the resin in light of production. Specifically, if the softening point of the resin is higher than the boiling point of the hydrophobic organic solvent, then pressure needs to be applied to a device for causing the resin composition to become particles.
In view of the problems, Patent Document 2 (Japanese Unexamined Patent Publication No. 2003-5443 (published on Jan. 8, 2003)) suggests a method of producing toner for realizing particles under a relatively low temperature.
According to the method of Document 2, toner is produced by (i) applying shear stress, in a hydrophobic organic medium to which the resin content is insoluble, to (a) a pigmented resin composition including a resin, an organic solvent, a coloring agent, and a charge control agent, and (b) a copolymer of polyvinylpyrrolidone and eicosyne so that the composition becomes particles to form toner particles, (ii) distilling off the organic solvent, and then (iii) separating the toner particle from the organic medium.
As described above, the dry method has a problem that it is difficult to further reduce the particle size of toner, whereas the wet method still has a problem of humidity-resistance, although the wet method can produce a smaller-sized toner than the dry method can.
In order to solve the above problems, Patent Documents 1 and 2 suggest adopting a method for producing toner that is excellent in humidity-resistance, in which method a pigmented resin composition is made to be particles in a hydrophobic medium with the use of a copolymer of polyvinylpyrrolidone and eicosyne, which copolymer acts as a dispersing agent. However, the toner of Documents 1 and 2 has a problem in that a surface of the toner becomes more adhesive due to a residual dispersing agent on the surface of the toner. This causes a tendency of toner aggregation.